What exports does blender have for 3d printers?
When considering 3d printing, you should be more concerned with your mesh topology than an actual output format. BlenderCookie has a tutorial on preparing a model for 3d printing. An application or company that does this will take anything from .ply to .obj as all these formats do is hold mesh data.

Here is also an extensive article on (recently new) official support for 3d printing in Blender.

So as for a definitive answer to this question. 3d printing giants Shapeways make mention of .STL in this tutorial


  Finally, export to STL. When uploading the file, make sure to select
  "millimeters" for scale. If all went well, it'll upload, and you'll
  get notified that it's ready for printing. 


and there is a set of tools called 3D Printing Toolbox available for Blender that assists with the overall process. Statistics/Cleanup/Export etc

You can enable the addon in Blender from the Mesh tab.



There is also a DVD on the Blender eShop on 3d printing. It is very modern and up to date.